A transient signal is a special audio signal, which often exists in an audio sequence produced by musical instruments including a percussion instrument. For instance, a signal produced by continuously striking the percussion instrument may be referred to as a transient signal. Such signal is characterized in that if the signal is encoded by a conventional transformation, such as Modified Discrete Cosine Transformation (MDCT), a pre-echo effect may occur due to the presence of the quantization noise. The pre-echo effect is caused by the quantization noise due to insufficient number of quantization bits. The quantization noise is distributed evenly into the whole time domain. The signal before the appearance of the transient signal may be occupied by the quantization noise and thus causing the pre-echo effect. Pre-echo effect is an audio distortion which human ears can hardly bear. Thus, there is a need for a special method for encoding or decoding a transient signal.
Two conventional techniques are available to process such transient signal. One is to switch between long and short windows, while the other is to perform noise rectification in a time domain. The switching between long and short windows requires a large amount of computational overhead and caches. The method of noise rectification in time domain rectifies the distribution of the quantization noise in time domain based on the result of self-adaptive estimation in frequency domain. This method is relatively simple, but may result in some distortions since the time-domain envelope is not extracted thoroughly.